1. Field of the Invention
The present invention relates to elastic wave devices used as resonators or band-pass filters and particularly relates to an elastic wave device making use of an SH plate wave propagating in LiNbO3 substrates.
2. Description of the Related Art
Hitherto, various elastic wave devices making use of a plate wave propagating in LiNbO3 have been proposed. For example, Japanese Unexamined Patent Application Publication No. 2002-152007 below discloses an elastic wave resonator shown in FIG. 16. The elastic wave resonator 1001 includes a LiNbO3 single-crystalline substrate 1002 as shown in FIG. 16. The center of the LiNbO3 single-crystalline substrate 1002 is a thin plate portion 1002a. That is, a central portion of the LiNbO3 single-crystalline substrate 1002 is thin and has a thickness h. In an example described in Japanese Unexamined Patent Application Publication No. 2002-152007, the thickness h is 0.03λ0.
The upper surface of the thin plate portion 1002a is overlaid with IDT electrodes 1003. Supposing the pitch between electrode fingers of the IDT electrodes 1003 is λ0, the width L of the electrode fingers is 0.125λ0.
Japanese Unexamined Patent Application Publication No. 2002-152007 describes that when the orientation of LiNbO3 is rotated about the X-axis at an angle of −10° to 70° in a direction from the Y-axis to the Z-axis, a major component of elastic vibration is an SH wave and a piezoelectric effect is large.
In the example described in Japanese Unexamined Patent Application Publication No. 2002-152007, the IDT electrodes 1003 are made of an Al-0.7% Cu alloy. On the other hand, the LiNbO3 single-crystalline substrate 1002 is a 40° Y substrate. This means that θ of Euler angles (φ, θ, ψ) is equal to 130°.
On the other hand, paragraph [0051] of Japanese Unexamined Patent Application Publication No. 2002-368576 below discloses an elastic wave element containing LiNbO3 with a rotation angle of 36°±20°, that is, LiNbO3 with Euler angles (0°, 106° to 146°, 0°). Herein, an elastic wave-propagating surface of a piezoelectric substrate made of LiNbO3 is mechanically free and the thickness of the piezoelectric substrate made of LiNbO3 is less than the wavelength λ of an elastic wave.
However, Japanese Unexamined Patent Application Publication No. 2002-152007 merely describes that when the orientation of LiNbO3 is within the above specific range, that is, a range specified in terms of Euler angles (0°, 80° to 160°, 0°), the piezoelectric effect due to the SH plate wave is large. A configuration using the SH plate wave described in Japanese Unexamined Patent Application Publication No. 2002-152007 cannot necessarily achieve a sufficiently large electromechanical coupling coefficient k2 and therefore is limited in increasing the fractional bandwidth, or bandwidth ratio. Incidentally, Japanese Unexamined Patent Application Publication No. 2002-152007 does not particularly refer to the thickness of the IDT electrodes.
Japanese Unexamined Patent Application Publication No. 2002-368576 discloses the elastic wave device containing LiNbO3 with the above Euler angles and a structure having an elastic wave-propagating region which is mechanically free and, however, does not describe any configuration using an SH plate wave.